A conventional raster-scanning type laser marking apparatus is adapted to scan a mask with a laser beam having a high peak power and to mark on a marking surface the shape of a part of the mask through which the laser beam is transmitted. However, since the laser beam is reduced so as to be narrow, its unit irradiation area is small.
Such laser marking apparatus, which provides a unit irradiation light having a high energy level, is effective for marking on a material having a relatively high marking threshold such as, for example, a metal. However, marking on a material having a low marking threshold such as, for example, a synthetic resin, is carried out on the same irradiation area by reducing this energy level. Therefore, despite a material having a low marking threshold, it requires the same marking time as a material having a high marking threshold, and the resultant efficiency is low.
For marking on the material having a low marking threshold, the energy can be maintained at the same level by increasing the irradiation area. However, if the area of the irradiated surface is expanded in an optical system of the conventional laser marking apparatus, a problem occurs in that satisfactory marking cannot be carried out because there are substantial differences in the distribution of the energy density between the central part of the irradiation area and the periphery thereof.